In this paper, a realization method is suggested for realtime distributed software-in-the loop (DSIL) simulation for distributed control systems. The realization is composed of several personal computers (PCs) for sub-plants and their controllers, an Ethernet network connecting them, and a general-purpose computer-aided control system design (CACSD) tool for the easy design of controllers. Requirements on real-time DSIL simulation are investigated in terms of network-related control issues and CACSD tool issues. CEMTool, a CACSD tool which is suitable for real-time DSIL simulation, is proposed for easy programming of control algorithm and plant dynamics and for their easy connection. To reduce a sampling interval and guarantee real time, a network scheduling aigorithm is suggested for real-time DSIL simulation. It is demonstrated via an experiment of turbinegenerator system that this real-time DSIL simulation is very useful for distributed control system design.